Add function test for path option

[chris5marsh]

This PR allows you to pass a function, which returns a string, to the path option. This allows you to change the path based on arguments sent through in the Grunt task.

e.g. running grunt stfp:cdn:dev and having as a path option:

path: function() {
  var type = grunt.task.current.args[0] || "dev";
  return "/path/to/my/cdn/"+type+"/";
}

I'm using it to upload to a different directory on our CDN based on the version number.